Smoking is the single largest preventable cause of mortality and morbidity in the United States, Regular smokers who attempt cessation often relapse, and treatment programs have had only modest success, which emphasizes the need for more effective smoking cessation techniques. Tryptophan has been used successfully to treat disorders that involve negative affect, such as chronic pain, insomnia, and depression, with minimal side effects. In addition, tryptophan increases brain serotonin, which could prevent increased food consumption and weight gains reported by smokers as a reason for relapse. The proposed study is a randomized, controlled trial to determine the effects of tryptophan as an adjunct to smoking cessation treatment. Subjects will be recruited from the general public who will attend smoking cessation group treatment at the Fred Hutchinson Cancer Research Center. Tryptophan (50 mg/kg/day) or placebo will be provided to subjects in a double-blind manner. Standard multicomponent cessation groups will consist of eight weekly sessions that include self-management strategies, knowledge and information, social support, cognitive restructuring, relaxation training, and relapse prevention training. The primary endpoint will be smoking status at one year after cessation. In addition, smoking status, withdrawal symptoms, patterns of food consumption, and weight gain during the group sessions and at regular intervals after the groups' end will be measures. Self-reported smoking status will be verified using biochemical indicators. If effective, tryptophan could be used as a relatively inexpensive, appealing pharmacologic adjunct for smokers who are trying to quit because of its potential control of withdrawal symptoms and weight gains and because of its minimal negative effects.